FIG. 6 shows a case where a set battery formed by connecting a plurality of secondary batteries in series is charged and discharged by a charge and discharge method for a secondary battery in the background art, and it is a graph showing, for example, changes of terminal voltages α11, α12, and α13 across three secondary batteries 111, 112, and 113, respectively.
Initially, at timing T101, the terminal voltages α11, α12, and α13 across the secondary batteries 111, 112, and 113, respectively, are all equal at a cut-off voltage of discharge, Vt (for example, 3.0 V) and the secondary batteries 111, 112, and 113 are in a balanced state. When a charge current is flown to charge the set battery, the terminal voltages α11, α12, and α13 gradually increase.
A secondary battery deteriorates when it is charged to the extent that the terminal voltage exceeds a predetermined cut-off voltage of charge, Vf. Accordingly, it is set in such a manner that in a case where the set battery as described above is charged, the voltage across the set battery takes a value found by: cut-off voltage of charge, Vf× the number of secondary batteries in series. The cut-off voltage of charge, Vf, is typically 4.2 V in a case where the secondary batteries are, for example, lithium-ion batteries. Hence, in the case of a set battery formed by connecting three secondary batteries 111, 112, and 113 in series, the set battery is charged until the voltage across the set battery reaches 4.2 V×3=12.6 V.
Incidentally, the internal resistance of a secondary battery increases when it deteriorates. Accordingly, when a charge voltage is applied across a series circuit formed by connecting a plurality of secondary batteries 111, 112, and 113 in series, the terminal voltage across a secondary battery having larger internal resistance, that is, a deteriorated secondary battery, becomes larger than those of the other intact batteries. When this happens, a charge voltage is no longer distributed equally to the respective secondary batteries. For example, assume that the secondary batteries 111, 112, and 113 are deteriorated more in this order, the terminal voltage α11 across the most deteriorated secondary battery 111 becomes the highest and the terminal voltage α13 across the least deteriorated secondary battery 113 becomes the lowest.
Then, at timing T102 in FIG. 6 when the charge ends, the terminal voltages α11, α12, and α13 across the secondary batteries 111, 112, and 113, respectively, are all different voltages, which gives rise to an imbalance among the secondary batteries 111, 112, and 113. By discharging the set battery that has been charged in the occurrence of an imbalance as described above by connecting a load to the set battery, then the terminal voltages across the secondary batteries drop faster in ascending order of deterioration.
In a case where a secondary battery is discharged, the secondary battery deteriorates when it is overdischarged. Accordingly, a voltage about at which the secondary battery will not be deteriorated is set as the cut-off voltage of discharge, Vt, which is the voltage at which discharge is to end. For example, in the case of a lithium-ion battery, the cut-off voltage of discharge, Vt, is typically set to a voltage of about 3.0 V. It is configured in such a manner that the discharge ends when the lowest voltage among the terminal voltages α11, α12, and α13 has dropped to the cut-off voltage of discharge, Vt=3.0 V (timing T103).
Consequently, at the timing T103, the terminal voltage α11 across the most deteriorated secondary battery 111 becomes the lowest and the terminal voltage α13 across the least deteriorated secondary battery 113 becomes the highest. An imbalance among the terminal voltages α11, α12, and α13 is therefore increased.
In addition, the terminal voltage α11 across the most deteriorated secondary battery 111 will have dropped to the cut-off voltage of discharge, Vt, before the terminal voltages α12 and α13 across the less deteriorated secondary batteries 112 and 113, respectively, drop to the cut-off voltage of discharge, Vt. The discharge is therefore ended and feeding of a current to the load is stopped. This causes an inconvenience that the capacities of the less deteriorated secondary batteries 112 and 113 are not utilized effectively and the capacity of the overall set battery is reduced.
While charge and discharge operations as above are repeated, as are indicated at timings T104 and T105, the secondary battery 111 more deteriorated than the other secondary batteries deteriorates further. This causes an inconvenience that a difference among the terminal voltages α11, α12, and α13 becomes larger.
Under these circumstances, there is a technique of reducing an imbalance among the secondary batteries forming the set battery by lessening a difference among the terminal voltages across a plurality of secondary batteries by forcedly discharging a secondary battery having the terminal voltage exceeding the cut-off voltage of charge, Vf, when the charge ends so as to lower the terminal voltage (for example, Patent Document 1).
FIG. 7 is a graph showing changes of the terminal voltages α11, α12, and α13 in a case where a difference among the terminal voltages across a plurality of the secondary batteries is lessened by forcedly discharging a secondary battery having the terminal voltage exceeding the cut-off voltage of charge, Vf, when the charge ends so as to lower the terminal voltage.
As is shown in FIG. 7, when the charge ends (timing T202), the secondary battery 111 having the terminal voltage exceeding the cut-off voltage of charge, Vf, is forcedly discharged for balance adjustment so as to lower the terminal voltage α11, a difference among the terminal voltages α11, α12, and α13 across the secondary batteries 111, 112, and 113, respectively, is lessened (timing T203).
According to the technique described in Patent Document 1, however, the most deteriorated secondary battery 111 is forcedly discharged further and the less deteriorated secondary batteries are not discharged. Hence, the chances of being discharged are increased more for the most deteriorated secondary battery 111 than for the less deteriorated secondary batteries, and the most deteriorated secondary battery is further deteriorated. This causes an inconvenience that deterioration varies considerably among the secondary batteries 111, 112, and 113 and an imbalance is increased.
In addition, as a secondary battery deteriorates further, the capacity decreases and the terminal voltage drops faster at the time of discharge. Then, according to the technique of Patent Document 1, because the secondary battery 111 that is considerably deteriorated and thereby has a reduced capacity is forcedly discharged further after the charge ends, an amount of electricity charged in the secondary battery 111 is reduced further. Hence, when the terminal voltage α11 across the most deteriorated secondary battery 111 has dropped to the cut-off voltage of discharge, Vt, due to discharge by use (timing T204), the terminal voltages α12 and α13 across the other secondary batteries 112 and 113, respectively, rather become higher than in a case where discharge is not forcedly performed for balance adjustment. This causes an inconvenience that there is a possibility that a difference among the terminal voltages α11, α12, and α13 across the secondary batteries 111, 112, and 113, respectively, is increased and so is an imbalance.    Patent Document 1: JP-A-2005-176520